1. Field of the Invention
This invention relates to scheduling means for data switching apparatus for use in computer-controlled digital data switching systems.
2. Description of the Related Art
Many types of data transmission apparatus are known, all having their own particular features and systems. In all cases the intention is to allow data switching and transmission to be achieved as rapidly as the apparatus will allow. It is common for data to be sent in “packets” consisting of a predetermined number of bits of data plus control information indicating certain parameters of the data or its mode of transmission.
In data switching apparatus having an number of users there may be a requirement at any one time to set up a number of different interconnections between input ports and output ports. In any form of switch there is a limit to the number of simultaneous interconnections that may be formed. The switch is operating at its greatest efficiency when the greatest possible number of interconnections is formed and switching apparatus frequently includes what may be termed “scheduling means” in order to achieve this maximum number of interconnections.
A good scheduling scheme needs to balance the potentially conflicting objectives of making sure that all output ports are connected where there are requests for a connection to that port (efficiency), that high priority traffic is serviced quickly (prioritisation) and that low priority traffic is not ignored (fairness). The present invention addresses all of these issues and may, for example, be used with the data switching apparatus described and claimed in our co-pending British patent application No. 9717412.2.
Various types of scheduling means are known. For example, U.S. Pat. No. 5,500,858 describes one form of scheduling means in which requests for interconnections are considered and satisfied using what are called “rotating priority iterative matching desynchronising scheduler units”. The “priority” in this case refers to priority given to input and output ports at any given time so as to ensure that each port has a fair chance of having a connection requests satisfied. The U.S. Patent goes on to describe how the scheme could be extended to handle requests at multiple priority levels but the scheme described would lack fairness, that is low priority requests would be ignored under heavy load conditions where only higher priority requests would be satisfied.